We have used the tritium suicide procedure to isolate two mutants of a mouse lymphocyte line which are defective in the "A System" of amino acid transport, and have used these mutants, along with the parental cell line, to characterize the "A System" as well as to discover and describe a system specific for dicarboxylic amino acids and cysteine. We propose to isolate additional mutants of mouse lymphocytes and of Chinese hamster ovary cells, both for the "A System" and the "L System" of amino acid transport; new methods of selection will be used, including selections for mutants that have either increased or decreased ability to form clones at minimal concentrations of essential amino acids. Temperature-sensitive mutants and, when appropriate, revertants, will also be selected. The gene products which have been altered by mutation will be analyzed for their roles in amino acid transport by comparing the activities and biochemical composition of membrane vesicles prepared from mutant and wild-type cells. Reconstituted membrane vesicles prepared from defective mutants will then be used as assay systems for the active components of wild-type membranes, prepared both from the parental rodent cells and from normal, human cells. Defective mutants of selected types will be fused with normal human cells and the hybrids will be used to map the human genes which restore the normal transport function.